Pulverized fuel slurries, such as pulverized coal-water slurries, are easily transported, stored and handled and, therefore, have found increasing use. The combustion of such slurries, however, presents problems because they contain significant amounts of liquids, in the case of coal-water slurries they contain up to 30% (by weight) water. Before combustion can commence, the water must be vaporized. This requires a significant amount of heat which must be quickly transferred to the slurry particles to initiate and sustain combustion.
The remainder of this application will primarily discuss and describe coal-water slurries. However, the invention is not so limited. It is equally useful for other slurries such as, for example, coal-methanol slurries, coal-methanol-water slurries, or coal-oil slurries, to name a few. The term "pulverized fuel slurry", therefore, is meant and should be understood to include all slurries made up of pulverized solid fuels suspended in a liquid.
In addition, the term "pulverized fuel slurry" refers to a slurry or slurries which may contain certain additives to change the viscosity of the slurry, maintain the solid particles in suspension, etc.
It is conventional to fire pulverized fuel slurries (hereinafter sometimes also referred to as "slurry" or "slurries") by atomizing them, that is by finely dispersing the solids-liquid particles into a combustion chamber and, thereafter, igniting them. A problem typically encountered during the atomization of such slurries is that the atomized particles have a tendency to agglomerate, that is to stick to each other when impacted in either the slurry atomizer or the combustion chamber downstream thereof. Once agglomerated, they are virtually impossible to separate.
Agglomerated particles have an increased mass which renders it more difficult to maintain them suspended in the combustion chamber. As a result, they can fall out without complete combustion, foul furnace surfaces and reduce the overall efficiency of the burner. Even if the agglomerated particles do not fall out, they require significantly longer stay times in the flame zone of the combustion chamber before they are completely burnt. Frequently, such extended stay times are not available, particularly in furnaces converted from oil or gas operation to slurry operation.
A further problem associated with slurry burners is the need for the rapid transfer of significant amounts of heat to the atomized slurry particles immediately downstream of the nozzle to initiate combustion. For example, gas burners require the transfer of approximately 1% of the total heat release of the burner to the incoming gas to generate a self-sustaining flame. Oil burners require about 1.5% and pulverized coal burners approximately 2% of the total heat to sustain the flame. In contrast, pulverized coal slurries with a water content of approximately 30% require the transfer of approximately 5% of the total heat release to generate a self-sustaining flame.
The heat required to evaporate the water is obtained from the main combustion chamber and the surrounding furnace walls. The heat transfer is enhanced by generating a low pressure core zone about the burner axis downstream of the nozzle which draws hot combustion gases rearwardly into a "recirculation zone." The prior art accomplished this by employing combustion air spinners which surround the nozzle and which spin the air at a more or less uniform rate over the entire radial extent of the spinner. The result of such a construction is that a low pressure zone is created at the center of the spinner which extends upstream into the spinner so that most of the air is emitted by the spinner at the peripheral portion thereof. A problem encountered with such prior art spinners is that the low pressure zone typically extends along the burner axis rearwardly to and past the nozzle, a problem which increases as the spin number is increased. As a result, recirculation gases contact the nozzle, often unacceptably heat it, and cause a fouling thereof which leads to inefficiencies, possible flame-outs and, in turn, substantial burner downtimes.
The heating of the atomized slurry particles is enhanced by widely dispersing them as, for example, by providing the nozzle with a large number of atomizing orifices. This is difficult to implement with slurry nozzles, as contrasted with oil atomizing nozzles, for example, because the relatively large solid particle sizes (typically in the range of between 0.003" to as large as 0.010") require relatively large orifice diameters and because the abrasive characteristics of solid fuel particles require the use of special, abrasion resistant material inserts, which limit the number of orifices which can be placed in the nozzle. Thus, there can only be a limited number of orifices, which must accommodate relatively large slurry flow rates. This increases the particle concentration in the atomized slurry cone downstream of the nozzle and thereby enhances the incidence of undesirable particle agglomeration.
Thus, there is at the present a need for an efficient slurry burner which achieves a self-sustaining flame without undue stay times for the atomized slurry particles and without fouling burner and furnace walls so that such burners can be used as a substitute for gas, oil and pulverized fuel burners in existing furnaces, for example.